Cerebral palsy (CP) is the most common pediatric neurological disorder. CP is caused by damage to brain motor areas during development. CP results in weakness, altered tone and abnormal coordination. Therapies for CP have largely been scaled from adult stroke therapies. However, the motor circuits involved in adult stroke are quite different from those in children with CP. Effective therapies for children with CP should be built on a specific understanding of motor circuit control in children with CP. We propose to identify the neurophysiological underpinnings of hand control in children with CP and will determine predictors of the efficacy of hand therapy. Building an understanding of hand motor control in children with CP fills an important gap and will set the foundation for further development of hand therapies for children with CP. Two types of intensive hand training have shown efficacy in children with unilateral spastic CP (USCP), whose motor deficits are largely restricted to one side of the body. In constraint-induced movement therapy (CIMT), the dominant arm is restrained while the impaired hand is trained in unimanual tasks. In hand-arm intensive bimanual training (HABIT), alternatively, children use both hands together. It has been reported that the efficacy of CIMT is affected by the pattern of corticospinal tract connectivity i children with USCP. The central hypothesis of the proposed research, supported by our pilot data, is that efficacy of hand training is differentially affected by connectivity of motor circuit and type of training. We also hypothesize that neurophysiological biomarkers of motor circuit dysfunction can predict which type of hand training is ideal for an individual. Two important questions remain unanswered: 1) How do motor circuits interact and change in response to intensive hand therapy? 2) Does the efficacy of intensive hand therapy depend on laterality of the corticospinal tract that controls the affected hand? The proposed experiments will answer these questions. Children are not little adults. Therapies for USCP must be built on a strong understanding of motor control in USCP. By identifying the neural circuit effects of CIMT vs. HABIT, we can tailor therapies to children most likely to benefit. This work will also provide a framework for developing new activity-based therapies for USCP.